Voicemail not accessible while roaming in the USA

I'm currently overseas in the USA. I am unable to access my voicemail - I have tried all of the following suggestions already:

#101# - message comes up as 'Not Registered'

+61 101 - get a 'this number is not available/ connected' message

101 - call will not go through

Responding with 'M' to the Telstra text telling me I have 2 new voicemail messages - message not delivered

Calling my own number - get a busy tone/unable to call this number message

Prior to the US, I was in Fiji for a few days and the voicemails were accessible from there.

I am on Telstra prepaid and purchased an International roaming pack prior to travel. Roaming is activated on my phone. And, as I mentioned, was working just fine in Fiji. No settings were changed between leaving Fiji and landing in the US.

Is there perhaps some difference in the agreement Telstra has with the mobile networks here? The carrier that I am getting service through is AT&T.